Prevention of crazing of flash heated shapes of polystyrene



Patented Aug. 12, 1947 PREVENTION or caazma F FLASH HEATED SHAPES OFPOLYSTYRENE Fred E. Wiley, Hartford, Conn., assignor to PlaxCorporation, Hartford, Conn., a corporation of Delaware No Drawing.Application May 4, 1943,

Serial No.

8 Claims.

Thi invention relates to the production of articles of polystyrene byprocesses which involve "flash heating which term as used herein meansan operation in which the surface of the polystyrene is rapidly heatedto or above, usually substantially above, the transition temperature ofthe material which is about 180 F. Flash heating may occur in bufllng,polishing or machining operations or in die drawing shapes ofpolystyrene in a highly heated die which may be heated to 580-600 F. Diedrawing may be used to. obtain articles of accurate shapes anddimensions and having a high polish.

Flash heating of shapes of polystyrene causes crazing or cracking uponcooling of the shapes.

I have discovered that crazing or cracking resulting from flash heatingof polystyrene shapes may be prevented by a certain heat treatment. Thistreatment involves heating a shape in a temperature range above thetransition temperature, preferably 194-221 F., for a certain period oftime before flash heating, then immediately flash heating the shape,then immediately quenching the shape in a cold bath, preferably a bathat a temperature below room temperature, for example, 4050 F., andholding the shape in the quenching bath for a certain minimum time. Theinitial heating of the shape is eflected in a a certain minimum time tosoften the sub-surface material but for an insuflicient time to softenthe core of the shape to such an extent that the shape can bepermanently deformed, or in the case of a rod, permanently stretcheduntil it'has become undersized as might occur in die drawing.

This treatment prevents surface crazing and cracking of the shape duringand after flash heating as well as preventing deformation or stretchingof the shape which might occur if the heating should be conducted at ahigher temperature and/or longer time.

Subsequent to the above steps the shape may be further treated tomechanically stabilize it, as by heating the shape to or above thetransition temperature, and then cooling it at a rate predetermined toproduce in the shape-a selected maximum stress which will permitmachining the shape or will permit it to be used without cracking orfailure, as a result of internal mechanical stress.

Example A rod of polystyrene is heated to a temperature of approximately194-221 F., preferably in a bath of ethylene glycol, for a period oftime not less than about one-quarter of the time necessary to obtainuniformity of temperature throughout the shape within 1 F. The length ofthe heating period is less than the time required to heat the core ofthe rod to such an extent that when the rod is subjected to flashheating in a drawing die and is drawn therethrough it would stretchpermanently and become undersized. After heating the rod within thelimits of time above stated, the shape is immediately flash heated as bydie drawing it through a die heated to 580-600 F. Immediately after theflash heating, the rod is quenched in a cold water bath which may bemaintained at a temperature of 40-50 F. The rod is held in the quenchingbath for a minimum period of time, preferably not less than one-half theapproximate time necessary to obtain uniform temperature throughout therod within 1 F. Following this treatment, the rod may be removed fromthe bath. The rod or other shape should not be allowed to remain out ofthe heating medium and the quenching bath any longer than necessarybefore and after flash heating and usually this period of time is notover 1 minutes.

It will be understood that changes may be made in the above describedprocedure and my novel process may housed in treating various materialscontaining polystyrene or the like, without departing from the scope ofthe appended claims.

Having thus described my invention, what I claim is:

1. The process of preventing crazing of a flash heated shape ofpolystyrene which comprises first heating the shape at a temperature ofapproximately 194-221 F., flash heating the shape at a temperaturesubstantially above said flrst named temperature, and quenching theshape.

2. The process of preventing the crazing of a flash heated shape ofpolystyrene which comprises heating the shape to a temperature ofapproximately l94-221 F. for a period of time not less thanapproximately one-fourth of the time required to obtain uniformity oftemperature throughout the shape within 1 F., flash heating the shape ata temperature substantially above said first-named temperature, andimmediately quenching the shape.

3. The process of preventing the crazing of a flash heated shape ofpolystyrene which comprises initially heating the shape at a temperatureof approximately 19422l F., flash heating the shape at a temperaturesubstantially above said first-named temperature, and quenching theshape in a bath maintained at a temperature of approximately 4050 F. a

4. The process of preventing the crazing of a flash heated shape ofpolystyrene which comprises initially heating said shape at atemperature of approximately 194-221 F. for not less than aboutone-fourth of the time necessary to obtain uniformity of temperaturethroughout the shape within 1 F. and for an insufficient period of timeto heat the core of the shape enough to permit permanent deformation ofstretching of the shape, flash heating the shape at a temperaturesubstantially above said first named temperature, and immediatelyquenching the shape.

5. The process of preventing the crazing of a flash heated shape ofpolystyrene which comprises heating said shape in a. liquid bath at atemperature of approximately 194-221 F. for not less than aboutone-quarter of the time necessary to obtain uniformity of temperature inthe shape within 1 F. and for a period of time less than that in whichthe core of the shape would be heated sufliciently to permit permanentdeformation or stretching, immediately flash heating the shape at atemperature substantially above said first-named temperature, andimmediately thereafter quenching the shape in a bath having atemperature not higher than approximately 50 F.

6. The process of preventing the crazing of a flash heated shape ofpolystyrene which comprises initially heating said shape at atemperature of approximately 194-221 F. for not less than aboutone-fourth of the time necessary to obtain unifprmity of temperaturewithin 1 F. for an insuflicient time for the core of the shape to beheated enough to permit permanent deformation or stretching of theshape, immediately flash heating the shape at a temperaturesubstantially above said first-named temperature, immediately quenchingthe shape in a bath maintained at a temperature not higher thanapproximately F. and holding the shape in the quenching bath for notless than approximately one-half the time necessary to obtain uniformtemperature throughout the shape within 1 F.

7. The process of preventing the crazlng of a flash heated shape ofpolystyrene which comprises first heating the shape to a temperature ofapproximately 194-221 F., immediately flash heating the shape at atemperature of approximately 580-600 F., and immediately quenching theshape 8. The process of preventing crazing of a flash heated shape ofpolystyrene which comprises first heating the shape at a temperature ofapproximately 194-221 F., immediately flash heating the shape by drawingit through a die heated to a temperature of approximately 580-600 F.,and thereafter immediately quenching the shape.

FRED E. WILEY.

REFERENCES CITED The following references are of record in the file ofthis patent:

UNITED STATES PATENTS OTHER REFERENCES Ind. & Eng. Chem., Sept. 1942,pages 1052 to 1056.

